1. Field of the Invention
The present invention relates to a scanner for scanning an image of an original, and a shading correction method for the scanner.
2. Description of the Related Art
A scanner provided in a copying machine and the like optically scans an image of an original by irradiating light to the original, and introducing reflected light from the original via an optical system composed of a mirror and the like into an image sensor in which photoelectric conversion elements such as a CCD (Charge Coupled Device) are arranged in multiple lines.
Such a scanner performs a shading correction for a scanned original image data in order to prevent the scanned image from being uneven caused by an unevenness of a light source amount, an irregular sensitivity of photoelectric conversion elements, and the like. In such a shading correction, a white reference plate provided in the scanner is scanned, and correction formulae to control an unevenness of output values between the photoelectric conversion elements are provided corresponding to the respective photoelectric conversion elements based on a reference image data obtained by scanning the white reference plate.
Practically, a shading correction is performed while scanning a black reference plate in addition to a white reference plate. In this description, however, an explanation about scanning the black reference plate is omitted.
In general, foreign objects such as dirt and dust are sometimes adhered to an optical mirror for introducing reflected light from an original into an image sensor. When locations of the foreign objects adhered to the mirror in a main scanning direction are not changed from when scanning a white reference plate for a shading correction when scanning an original, the shading correction is appropriately performed.
Meanwhile, a carriage mounting the mirror may be shifted in the main scanning direction for some reason when the carriage is moved in a sub scanning direction to scan the original. In this case, the locations of the foreign objects are to be shifted in the main scanning direction when scanning the original.
Thus, the shading correction in the locations of the foreign objects is not performed properly. Accordingly, in an output image based on a scanned original image data, white lines along the sub scanning direction are caused at locations corresponding to the locations of the foreign objects in the main scanning direction when scanning the white reference plate. Moreover, black lines along the sub scanning direction are caused at the locations corresponding to the locations of the foreign objects in the main scanning direction when scanning the original.
Even when the carriage is not shifted in the main scanning direction during moving, the similar improper phenomena as described above are caused in the output image when the foreign objects are moved on the mirror in the main scanning direction between the scan of the white reference plate and the scan of the original.
Then, Japanese Patent Laid-Open Publication No. 2006-270604 discloses a method for smoothing local drops of an image data value when the local drops in the image data value regarding a white reference plate is detected, and performing a shading correction by use of the smoothed image data, so as to suppress quality deterioration of an output image irrespective of whether foreign objects adhered to a mirror are moved or not.